The question demands me to find "rupture lifetime" for 300MPa load at a temperature of 649$^{\circ}$ C.How do I solve it?All that I know is to apply interpolation and extrapolation which I am basically unabale to apply.Also if I am given temperatues like 700$^{\circ}$ C or 300 $^{\circ}$C then is it possible to solve?
Assuming the curve for $649 ^\text{o}$C is linear you can obtain the equation of the line passing through $(10^2, 80)$ and $(10^5, 20)$. This turns out to be: $$\ \text{Stress(MPa)} = -20(log_{10}(L)-6) \text{ where } L\to \text{Lifetime in Hours} $$ Note: The relationship is linear between Stress and $log_{10}$ of rupture lifetime.
I don't see any way to compute for temperatures like $700 ^\text{o}$C or $300 ^\text{o}$C. Maybe you can try interpolation and extrapolation for different temperatures at a constant Stress value but I highly doubt if it will be correct as you just have data for just three temperature values.